Known for many decades, multiple hearth furnaces have been used for carrying out, as continuous processes, a range of thermal treatments on an extremely wide range of bulk solid solid materials. These include:                Incineration and pyrolysis, for example of biosolids from municipal wastewater (that is, sewage) and industrial sludges;        Calcination, for example of Carbonate materials such as Calcium Carbonate in cement manufacture;        Drying and dehydration, for example in treatment of bauxite and gypsum;        Roasting of mineral ores, for example many sulphide ores; and        Regeneration and recovery, for example of activated carbon, and foundry sands,        to name only a few process/material combinations.        
The basic multiple hearth furnace arrangement has proven very versatile and has been used over time with a wide range of external solid- and gas-handling components and circuits. Improved design methods and improvements in instrumentation and control have also contributed to their widespread adoption.
In recent years, environmental requirements and energy efficiency have become progressively more important. Sometimes these factors have led, wholly or in part, to adoption of other furnace types in new installations. They may constrain use of multiple hearth furnaces in some applications, despite the many advantages of the type.
Further, there are substantial numbers of existing multiple hearth furnace installations, and there is a desire by users of many of these for ways to upgrade their performance to deal with environmental constraints and fuel efficiency requirements. An additional issue in upgrading of furnace installations may also be a need or desire to operate at higher solids throughputs, due to increasing local population.
Processing of biosolids from municipal wastewater (sewage) provides an illustrative example. Multiple hearth furnaces have been used in this application at least since the 1930s, and for many years were the preferred choice. However, more recently, alternative furnace types such as fluid bed furnaces have been adopted in significant numbers in new installations due to increasingly stringent regulation of gas emissions and due to increasing fuel costs. Operators of multiple hearth furnaces subject to capacity, environmental or other constraints often seek ways to improve them as an alternative to the capital costs of installing replacements or new plant types. There is also a desire for design options that can produce more cost-effective and efficient new multiple hearth furnaces.
Various avenues for improvement of multiple hearth furnaces have been explored, including for example the use of internal and external afterburners—these can be helpful in reducing unwanted emissions, such as Carbon Monoxide (CO), but at the price of increased fuel costs.
The present invention therefore addresses the desire for additional options in the design of new multiple hearth furnaces and for improvements of existing ones and is particularly directed to control of gas residence times. These can be important in the control of emissions, and fuel efficiency, as well as the achievement of close control of changes to the solid material being processed.
The present invention was motivated by the biosolids incineration application, but it is thought that it may have application to other applications of multiple hearth furnaces.
The invention relates to the use of annular baffles above hearths of multiple hearth furnaces. The use of one or more annular baffles in multiple hearth furnaces has been proposed in only two disclosures known to the inventors, namely related U.S. Pat. Nos. 4,626,258 and 4,728,339. However, the present invention is quite different from these disclosures. The combined conical and annular baffles described in these early patents are intended to increase the intimacy of solids/gas contact and appear to reduce, rather than increase average gas residence times above hearths. The annular baffles have a diameter less than that of the central drop holes, so that an entirely different flow pattern would be developed—and the flow pattern developed in the presence of baffles is important in the present invention. There is no disclosure of annular baffles on “in” hearths (as defined below) nor is there disclosure of the use of annular baffles without cooperating conical baffles.
Baffles have been proposed in single hearth furnaces, for the purpose of enabling essentially two-stage treatment processes to be carried out on one hearth; see for example U.S. Pat. Nos. 3,448,012, 4,637,795, and 4,741,693. These show single hearth furnaces in which there are two distinct annular treatment zones, separated by a cylindrical baffle depending from the furnace roof over a hearth. However, these differ from the present invention in that their intention is to distinctly separate the two treatment zones, through minimizing any gap between the baffle lower edge and the material on the hearth immediately below, and so limit leakage of radiant heat and hot gases between the zones. The present invention flows from the surprising discovery that despite the rabble arms imposing a limitation on how small the gap between feed material and baffle can be, there is nevertheless an advantageous effect, albeit in a different type of application.